In give and take of evolution, a surprising contribution from islands

Islands hold a special place in the hearts of evolutionary biologists. When Charles Darwin visited the Galapagos Islands in 1835, he was stunned by the diversity of birds, which helped guide him to his theory of evolution by natural selection.

Beginning in the middle of the last century, the ornithologist Ernst Mayr laid the foundation for the modern understanding of the way new species evolve, arguing that they mainly emerged when populations became geographically isolated. Mayr based his theory on his studies of birds from Pacific islands.

Yet islands have generally been considered evolutionary dead ends. After animals and plants emigrated from the mainland, it was believed that they became so specialized for island life that they could not leave. They eventually became extinct, only to be replaced by new arrivals from the mainland.

''They were like baubles of the evolutionary past,'' said Christopher E. Filardi, a biologist at the American Museum of Natural History.

But Dr. Filardi and Robert Moyle, a colleague at the museum, have found evidence that islands can act as engines of evolution instead of dead ends.

Animals can spread from island to island, giving rise to an explosion of new species, and even colonizing the mainland again. The results suggest that conserving biodiversity on islands is vital for the evolution of new species in the future.

Other recent studies suggest that islands may also be engines of evolution for many other animals and perhaps even plants. In the June issue of The Journal of Biogeography, for example, Kirsten Nicholson, postdoctoral research associate at Washington University, and her colleagues published a study of lizards that live in Central and South America.

The team demonstrated that 123 mainland species are the descendants of an ancestor that lived in the West Indies.

•	In Give and Take of Evolution, a Surprising Contribution From Islands The New York Times, Tuesday, Nov. 22, 2005 Byline: Carl Zimmer